The U.S. Congress has before it an immigration bill that represents a large step towards long-overdue immigration reform. Among its virtues, the bipartisan McCain-Kennedy bill would put more teeth into border enforcement measures and introduce more humane treatment of illegal immigrants already living in the U.S. It will take a balanced reform of this kind to quell a simmering demographic division in the U.S., which is affecting the politics of immigration in ways that seem at cross purposes with the future economic security of senior citizens.

Outsiders must see Americans as downright muddled over their demographic destiny. The White House is in crisis mode over the impending imbalance between the wave of baby-boom workers retiring and the much smaller numbers of workers, who will be asked to subsidize Social Security. Yet, there have also been volunteer "Minutemen" patrolling the U.S.-Mexico border trying to prevent immigrants getting to the U.S., even though immigrants bolster the young, working-aged population.

These contradictory views are likely to take on a regional dimension if current immigration flows continue undeterred. This is highlighted in the Census Bureau's recent 25-year population projection. It portrays an increasingly sharp demographic divide between slow-growing, immigrant-starved states that will undergo "extreme aging" and those that will grow more rapidly, in large part from immigration.

If any part of the country needs to be worried about elderly support, it will be a projected "Grey Belt" spanning Montana through to New England, which will come to resemble the more geriatric countries of Europe. Due to their "brain drains" and inability to attract immigrants, states such as West Virginia and Maine will see their median ages rise from 39 to 47. In 2030, 10 states will have more retired people than children. These states could surely use an infusion of young immigrants, if for no other reason than to contribute to exploding elderly support services.

In contrast, the projection depicts a slew of growing southern and western states, where immigration represents much of the gain. Owing to youthful newcomers, from abroad and the rest of the U.S., their median ages will rise only modestly over the next 25 years: to about 35 in Texas and Georgia.

These states also have aging baby boomers, but their policy focus is more attuned to the younger migration dynamic and, in particular, illegal immigration. This is especially true in the new destination states, where numbers of illegal immigrants are growing and where media-driven concerns over national security, crime and culture have aroused anti-immigrant reaction and initiatives to deny public services to illegal immigrants.

Yet, a patchwork of random state laws cannot be a substitute for a federal immigration policy that recognizes the realities of the nation's unbalanced demography. One of these realities is the 11 million illegal residents who are part of established communities and play key roles in a number of industries. It is both unfeasible and, I dare say, un-American to consider any mass deportation effort. However, it is also a reality that these illegal immigrants are largely unskilled and geographically concentrated within a handful of states that disproportionately bear prime responsibility for their education and health services. Their concentration in these states is a result of the clandestine migration networks they are forced to follow.

The current policy, which admits legal immigrants mostly on the basis of family reunification and, to a lesser degree, to take jobs "that Americans won't do," will not serve the economic needs of an aging society.

A forward-looking immigration policy needs to bring illegal immigrants out of the shadows by giving them a path towards earned citizenship, as well as geographic and job mobility to all parts of the country; provide realistic quotas and mechanisms (including tough employer sanctions) to ensure that future immigrants are legally registered; and revise rules for admission by placing greater emphasis on skills and work experience. The McCain-Kennedy bill goes a long way towards achieving the first two objectives. But the third is just as important. Maintaining the current broken system is not a viable option. Its continuation would probably lead to the demographically and politically balkanized nation that the Census Bureau's projection implies.

The writer, a senior fellow at the Milken Institute, a demographer at the Brookings Institution and a research professor at the University of Michigan Population Studies Centre, is author of "America by the Numbers" (The New Press)